Category Archives: Landslide

Geosysta at 12th International Symposium on Landslides (ISL)

Landslide

Training and professional development have always been integral to operations at Geosysta. It is our priority to keep our team learning new skills and maintain our strong connections with academia and professionals by always trying to share our knowledge in the fields of geotechnical and mining engineering.

As part of our constant evolvement, Geosysta participated in the 2016 12th International Symposium on Landslides (ISL) held 12-19 June in Napoli, Italy, where well documented case histories of landslides were presented providing a better understanding of related mechanics, accounting for relevant soil and rock properties and their behaviour.

heade_new_06-05-2015

As per the conference committee: “The landslides risk has strongly increased over the last decade, mainly because of ever growing population and relevant bigger exposure. In many countries, this is also due to expanded civil and industrial settlements, as well as widened infrastructures and lifelines. For this reason, in order to perform risk analysis and management, it is necessary a better understanding of landslides’ mechanics, accounting for relevant soil and rock properties and their behaviour in well documented case histories.”

Sessions that focused on landslides development mechanisms and the findings of post geotechnical investigation, moving from experience to practice through theory, were presented during the conference. The conference’s presentations covered a wide range of both practical and theoretical approaches when investigating the reasons resulting to landslides and how their findings could be used in order to predict similar future events.

Georgia Papavgeri, Civil and Geotechnical Engineer at Geosysta, said: “Everything came together for a successful conference. It was exciting to be among so many experienced professionals being able to share Geosysta’s unique story and expertise on landslides events”.

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Georgia Papavgeri at ISL 2016

Georgia presented a 10 minutes session on “Large moving landslide inside a lignite mine in Northern Greece”. The presentation discussed an actual case inside an 170-180-meter deep lignite mine in Northern Greece, where the combination of thin clay layers, overlaying deep sandy layers with an unfavorable inclination and complex tectonic formation have resulted in a slow moving landslide. Since this is an active mine, with large earthwork moving operations, it was important to accurately evaluate the mechanisms that led to the formation of the landslide, and the effects of the moving landslide body to the operations of the mine. During the investigation of this large landslide area, different data were utilized, starting from borehole data of more than 50 borings ranging from 150-300m deep, 6 borehole in which High Resolution Acoustic Televiewer loggings where performed, and three deep inclinometers. All data were combined and critically evaluated to generate the possible landslide mechanisms and to evaluate different analysis methodologies.received_881193118652763

Another important component of ISL 2016 was the trade show and the participation of many sponsors from across Italy, Europe and around the world. During the Symposium there was the opportunity for Exhibitors, Producers and Distributors, Public and Private Societies, Contractors and Consultants to promote the name and the image of their companies to all registered participants.

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Yeager airport landslide

Yeager Airport Expansion slide additional comments

Geotechnical information, Landslide, , ,

In my previous entry regarding the Yeager airport landslide where I hypothesize for a possible shear zone somewhere near the foundation, I got some interesting comments from fellow engineers. In this entry I would like to clarify some issues.

The hypothesis made about a shear zone was based on published information regarding intercalation of sandstone and shale and photograph observations of the area. A possible dormant landslide was hypothesized by other contributors in the ASCE group in LinkedIn. The model I presented was very simplified in order to test the possibility of the shear surface hypothesis. I will come back to the model with some more details, but I wouldn’t like to deviate from the failure discussion to a model discussion.

I stated previously that usually failures of such large magnitude are more complex and significant data and observations are required to understand the real mechanism. This I am sure will be dealt with by enquiry committees, forensic consultants etc.

I based my hypothesis considering that the Designers of the reinforced earth had done a good job, that the materials used were of appropriate quality and that something outside the structure was responsible or partially responsible for this failure. Many have commented regarding the reinforcement design and all are valuable comments which I will not reply but can be found here. Before a detail analysis and modeling of the failure is possible, additional hard data are required, such as shear strength properties of the actual soil, current tensile strength of the geogrid (as found in the landslide area), excavation at the toe of the slope etc. Until such data become available, only hypothesis can be made that may be completely wrong in the end.

The reason why I made the hypothesis of a shear zone is because it is based on previous information about older landslides in the area, because shale is notoriously tricky material when combined with sandstone and to broaden the possibilities of failure outside the earth structure.

Many times, failures are formed due to very thin weak layers which are very difficult to identify during ground investigation. Sometimes zones (or layers) of a couple of centimeters can be responsible for extended failures. Such zones many times are ignored, especially in very large structures. Consider a borehole of 50-100m with a low strength shale zone of a couple of centimeters, is it always possible to identify it? Even if not ignored, during investigation, a shale zone could appear strong and competent. In the following photograph a translational failure on a lignite mine can be observed. The failure took place on a clayey shale layer of couple of centimeters in a nearly horizontal stratification material. Observe the magnitude of the failure in relation to the huge bucket wheel excavators. Also observe the horizontal movement based on the misalignment of the conveyor belts. The slope inclinations before failure were very shallow, around 1:3 (V:H).

Mine landslide Geotechpedia
Mine landslide

Coming to the slope stability model I used, it was only to validate the possibility of such a failure and not to model the actual reinforced earth slope and its failure. The parameters used were the ones provided by the Lostumbo 2010 presentation and instead of including the reinforcement, a cohesion was used to produce a factor of safety above 1.3 for a circular shear surface inside the soil reinforced structure. A shear surface was not included and a factor of safety above 1.3 was selected because it is assumed that the structure would have been designed above this FS. It is not the intention to assume a soil material with such cohesion. The initial calculation to validate the stability of the model before the failure surface is presented in the following figure.

Slope model
Slope model

But once again I hope the discussion will not deviate from the actual issue and get focused on the model. As R. Peck very elegantly noted “stability analyses are tools for the guidance of the investigator. They have their limitations with respect to evaluating the stability of existing dams [the paper was about dam failure] It is not meant that they should never be performed. However, the numerical values for the factor of safety should carry little if any weight in judging the actual safety of the structure with respect to catastrophic failure”. Peck was evaluating a dam failure, and he focused on other issues that play important roles in relation to failures. So in that context I (among others) proposed the lower shear failure or old landslide issue as part of the controlling factors. Hopefully soon we will have many additional hard data to address this issue.

Finally I would like to note that earth retaining structures are a very good solution to many situations and we should not be reluctant to use them because of such incidents. We should though learn about such failures and put all our effort to avoid them in the future.

References:

R. Peck, (1998). “The Place of Stability Calculations in Evaluating the Safety of Existing Embankmnet Dams”, Civil Engineering Practice, Fall 1998.

Yeager failed slope

What could have gone wrong in Yeager Airport Expansion slide?

Geotechnical information, Landslide, , ,
Yeager airport landslide
Yeager airport landslide

Recent news and photographs present the spectacular slide that occurred in the Yeager Airport Expansion Runway 5. The slide occurred in the South slope which was among the highest if not the highest (~74m) reinforced earth slope in the US. The project had received the award of Excellence – TenCate Geosynthetics in 2007 International Achievement Awards. According to FHWA Manual the Yeager Airport in Charleston WV had been constructed as a massive earthwork in 1940’s. The mountainous conditions around the airport produced steeply dipping slopes to the Elk and Kanawha Rivers. In order to meet FAA Safety Standards runway 5 required a 150m extension in order to create an emergency stopping apron. The most cost effective solution was a 74m high 1H:1V reinforced steepened slope (RSS). The chosen solution is presented in Figure 1 taken from Lostumbo 2010.

2010 STGEC - Yeager Airport - Tallest Reinforced Slope in N America
Figure 1: Reinforced 1:1 earth slope extension of runway 5

As can be seen from Figure 1, most of the reinforced earth area is above the original ground and only a small part in the slope base is excavated in order to found the reinforced earth structure. Based on Lostumbo, 2010 over 100 borings were performed, with extensive laboratory testing and the final outcome was that the site consisted of primarily fill, colluvial and shallow rock. The material parameters used for the bearing soil zone were unit weight γ=22kN/m3, φ’=40ο and c’=0kPa. I would like to provide some speculations regarding this incident based on the available data found on line and photographs from the news and Google earth. I must point out that these are only speculations since no official data are available to me, nor the exact design or construction plans. These speculations are made just by observations and engineering imagination!   I am sure that significant investigation will take place in the coming months and years which will produce the actual conditions and reasons for this spectacular failure. A picture from Google earth taken on 9/2005 presents the initiation of excavation for the construction of the reinforced earth slope.

Yeager SW slope copy
Figure 2: Google earth image of excavation at Southwest slope of runway 5

A newer picture taken on 4/2006 presents the progress of works in which a part of the reinforced earth slope has been constructed.

Yeager airport partially constructed slope
Figure 3: Google earth image with partially constructed slope

It is very interesting to note that the excavation and foundation of the earth structure did not go all the way to the base of the hill. It is possible that good foundation material (assuming rock) was found in some elevation and was considered appropriate for founding the structure. After all the large earth structure is placed on sound rock (mostly sandstone) with a compressive strength between 30-95MPa! Weathered sandstone from the borrow area had a friction angle between 38.9-39.6o. The placement of the reinforced earth structure on top of bedrock can be observed in figure 4. The bedrock is clearly visible in the back and some moisture can be observed a bit higher in the slope.

Is the “competent” foundation bedrock to blame?, is the design of the reinforced slope to blame? Is the construction practice? Is the intense rainfall? Usually many factors contribute to such a large failure, but at this point with very limited information I would like to focus on the bedrock conditions and the fill material placed on the bedrock.

Yeager airport reinforcement placement
Figure 4: Site photograph shown the placement of reinforcement near the base of the slope, rock formation is clearly visible in the back (Lostumbo 2010).

Is a sound bedrock always appropriate for placing such a large structure? Based on the unconfined compressive strength I would say yes, but are other conditions at play here? Based on Huang et al, 2014 “the on-site geomorphology consisted of weathered sandstone underlain by sandstone and some shale.” and “The compressive strength of the rock foundation varied from 30MPa to 95MPa. The high bearing capacity of the underlying sandstone foundation and the high friction angle of the onsite weathered sandstone soil meant that the extent of the reinforced slope could be kept to a minimum, and maximum use could be made of the onsite soil.”

Notice the phrase “sandstone and some shale”. Could this simple phrase be the key for what happened? It is well known that even small intercalations of shale can produce enormous geotechnical problems as was the case of Landslide on No.3 Freeway in Taiwan (Duncan, 2013). The first reason is that shale materials have much lower compressive strength but more importantly considerably lower friction angle. Furthermore if not fractured, they present a very low permeability barrier. Usually water is seeping in the sandstone – shale interface, asymmetrically weathers the shale and also produces increased pore pressures in that interface. Could such an interface (or failure surface) had been formed in this case? The answer is, it may be possible and can be seen in the following very simple model in figure 5. (will not go into much detail about the model it is just an example of the possible formation of such a failure surface).

Yeager airport slide model copy
Figure 5: Very simple model evaluating the possibility of failure due to weathered shale intercalation

Now let’s go back to actual observations, figure 6 is a Google earth image taken on 3/2012. Please observe the stones between the slope and the road in the red circle. Then let’s go to Google street view in the same location and what we see is shown in figure 7. The layering of the bedrock is clearly visible, furthermore some form of fissure can be observed even with some horizontal movement one can argue based on this image. Could such a feature or a similar one in different elevation be the weakest link of this structure? Could it have been in marginal stability and all it took was some heavy rainfall that increased the pore pressures in this interface and initiated the slide? Food for thought until the actual investigation comes out and the real conditions that lead to instability, which I really hope are much more complex, can be addressed.

Yeager airport google earth 1
Figure 6: Google earth image before the failure, note the rocks between slope and road
Yeager airport toe slope possible failure surface
Figure 7: Google street view in the road just where the rocks are seen. Observe a possible shear surface.

References:

Duncan J. M. (2013). “Impact of time on the performance of reinforced slopes” Geo-Congress 2013.

Huang Z., Al-Saad Q., Nasrazadani S., Wu Felix H. (2014). “Understanding and optimizing the geosynthetic-reinforced steep slopes“, EJGE Vol. 19, 2014

Mohr – Coulomb failure criterion continued

Landslide, soil mechanics, ,

Things to remember when using the Mohr – Coulomb failure criterion:

  • The linear failure envelope is just an approximation to simplify calculations
  • The failure envelope is stress dependent and will produce some kind of curvature if shear strength tests are executed in much different confining stresses (fig 1, from Duncan and Write, 2005).

 

  • According to Lade, 2010 the failure envelope is curved and at low effective stresses which can be found in superficial failures on slopes, the use of linear Mohr – Coulomb may be in the unsafe side. Soils without cementation do not provide any effective cohesion in very low effective stresses (fig 2, from Lade, 2010).

 

  • When the linear Mohr – Coulomb criterion is used it must be evaluated for the expected stress range in the field.
  • Small cohesion values will not produce significant errors when high effective stresses are anticipated in the calculation.
  • In low effective stress even minimum values of effective cohesion (in cohesionless soils) can produce significant errors in factor of Safety (FS) calculations.

References:

Duncan J. M.,  Wright S. G., (2005). “Soil Strength and Slope Stability”. Wiley, New York.

Lade P. V. (2010). “The mechanics of surficial failure in soil slopes”. Engineering Geology 114, pp 57-64.

Slope stability and scale effects

Engineering Geology, Landslide, ,

In previous entries the issue of stiff fissured clays and the time to failure was briefly touched. The design of such slopes is not a trivial matter and requires significant knowledge of soil mechanics, geology, hydrogeology etc. One additional issue mentioned (one that sometimes is neglected) is the scale effect. This was presented in the previous entry for a very deep mine in rock. This issue of scale effect in relation to stress field will be briefly presented for the case of stiff fissured clays and hard soils.

In the following picture a large highway cut of about 30m is shown. For a civil engineering project this is a significantly high cut. The effective stress filed in this cut can range from of 50 – 500kPa which is the normal range for laboratory testing.

Highway cut

In the second picture a large excavation for a lignite mine is presented. The depth of excavation of this multi bench cut is around 135m. The excavation of this type needs to consider bench stability of slopes with heights of around 18m and also overall slope stability for highs above 135m. In the second case a large part of a possible failure surface could be in a stress field of around 1500-2000kPa or even more.

Coal mine slopes

In the following figure the two types of cuts are compared and one can easily understand the significance of scale effects in the design of the different cuts.

Scale difference of coal mine and highway slopes

The scale of the mine excavation is such that even in one cross section, one has to consider besides the stress field, differing geology (pic 4), presence of faults, ground water locations and pore pressures etc. We will focus on the stress dependency at this point.

mine slopes

According to Stark et al, (2005) both fully softened and residual failure envelopes are stress dependent. In this work Stark et al provides an empirical graph regarding the stress dependency until 700kPa of normal stress for residual friction angle and 400kPa for fully softened friction angle.

Shear strength information for higher effective stresses >1MPa are not readily available. Furthermore execution of such tests in very high effective loads is not easy for most commercial laboratories. It may even be very difficult to execute ring shear tests in very high loads due to sample thickness and squeezing out from the sides.

In such high slopes the failure surface can pass from a number of soil layers with different shear strength properties. It is not easy to evaluate the “average” shear strength of layers involved in a possible failure surface. Unfortunately a rule of thumb for selecting shear strength parameters for such slopes cannot be provided. Engineering judgment is required in selecting such parameters and the stress conditions must not be ignored. Shear strength tests should be evaluated in relation to the expected stress field.

Slope failures, landslides and mines

Geotechnical information, Landslide, Site Features, ,

On 11 of April 2013, around 9:30 p.m. a large slide (maybe the largest) in the northeast section of the Kennecott mine occured (fig 1). The slide was preceded by slope movements that reached ~50mm per day. Two major questions could be raised, why this slide occurred and could it have been predicted before hand and remediated?

Kennecott mine

These are very difficult questions and require significant knowledge of the geology, geotechnical conditions of the area, operational practices, climatic conditions etc. In the following paragraphs some initial ideas regarding the stability of high mine slopes and some interesting references will be provided for interested individuals. The incident in Kennecott is an important lesson of how important continuous monitoring of slopes is in such mine operations.

I would like to start with a very interesting graph published by Hoek et al (2000), “Large-scale slope Design – A Review of the State of the Art”.

This chart presents slope height versus overall angle with solid markers representing unstable slopes and open markers represent stable slopes. This chart is for copper porphyry open pits. In this graph the Kennocott mine (Bingham Canyon) is also shown but not the April 2013 one.

It is very interesting to note that most of the unstable markers are located in a range between 35 and 45 degrees of slope angle. Although much information is required for detail evaluation of each point and why instability occurred, a trend can be seen. Can we assume that slopes designed bellow 32-35o would not provide stability problems?

In the next figure I would like to focus on scale effects when dealing with mine slopes in rock or even hard rock materials. In the down left side of the figure 2 a slope with 30 meters height is depicted. In the upper left one of 90m with the same spacing of joints and finaly on the right a slope of 500m again with the same spacing and trance length of discontinuities (figures adopted from Sjoberg, 1996).

What can be seen from this slope scale is that even solid lightly fractured hard rock can be seen as an accumulation of infinite rock items such as a gravel slope or sand slope, just with better interlocking. One additional question can be, “what is the effect of bridging (intact rock between discontinuities) in such large scale slopes”? Very difficult question but maybe the previous graph provides an explanation (maybe negligible?).

Is the scale effect, in relation to joint spacing, orientation and stress field producing a ductile (sand or gravel like) behavior that may control the overall stability?

In the left graph (Ross, 1949) tests on intact marble with different confining pressures are presented. On the right (Holtz, 1981) normalized stress – strain with different confining pressures for dense Sacramento sand are presented. As can be seen, both materials in low confining pressures present a brittle behavior and as confining pressure increases, the behavior becomes more ductile and strain hardening.

Strong rock and dense sand can have the same behavior in different stress scale? And if this is the case, should high slopes be treated in a different way? Neglecting cohesion and using a possible “critical state friction angle” approach in slope stability? This issue requires additional research and detailed case studies but at least we can have some perspective regarding to scale effects.

References:

Sjoberg J., (1996). Large scale slope stability in open pit mining – a review. Technical Report 1996:10T, Lulea University of Technology

Hoek E., Rippere K. H. and Stacey P.F. (2000). Chapter 1, Slope stability in surface mining, Hustrulid, McCarter, VanZyl (eds), Society for Mining, Metallurgy and Exploration

Ros Μ. und Eichinger Α. (1949). Die Bruchgefahr fester Korper (Eidgenoss. Material prufungs versuchsanstalt, Ind., Bauw. Gewerbe. ZUrich, l72), 246 pp.

Holtz R. D., Kovacs W. D., (1981). “An Introduction to Geotechnical Engineering”, Prentice Hall.

How long can stiff fissured clay slopes stand?

Landslide,

Coming back to the issue of stiff fissured clay slopes, one very important question is the standup time of a slope excavated or formed (natural erosion etc) with higher inclination than the one that would be the outcome using fully softened shear strength.

Skempton (1970) in his paper “First time slides in over consolidated clays”, provided a graph presenting the decay of strength with time to almost fully softened where failure of slopes in London clay occurred (pic 1).

Strength changes with time for London Clay (from Skempton, 1970)

Mesri et al (2003) in his paper provides a graph (pic 2) compiled from numerous case studies of failed slopes in stiff fissured clays in which the vertical axis presents the percent of failure surface length (Lr) in residual condition to the total observed failure surface length (Lt)  and on the horizontal axis the age when the slopes failed. The far left points are of unidentified time. Time to failure range from a couple of years to decades.

Ratio of slip surface segment assumed at residual condition to total slip surface length ploted against age of slope

VandenBerge et al (2013), mention that “The time required to reach fully softened strength might be as little as ten years in some cases, and as long as 60 years in others”.

Potts et al (1997) in the paper “Delayed collapse of cut slopes in stiff clay” presented complex coupled finite element analyses assuming strain softening soil behavior capable of swelling. The analysis requires among other parameters the coefficient of permeability which varies with depth, and coefficient of earth pressure at rest. With this complex parametric analysis they conclude that 3:1 (H:V) slopes produced failures between 11 and 45 years. And steeper slopes fail in less than a decade. 15m high slopes failed after 145 years. All these slopes produced deep seated progressive failure.

It is very interesting to note that they evaluated deep seated failures although the problem of investigation was recent shallow failures in highway cuts and embankments.

Another attempt from Kovacevic et al (2007) presented in the paper “Predicting the stand up time of temporary London Clay slopes at Terminal 5, Heathrow Airport” in which again a complex finite element analysis with swelling behavior and differing Ko was executed. The investigation was for both shallow and deep seated failure. The conclusion of the study was that shallow failures occurred earlier than deep seated and the time to failure was influenced most by the assumptions of permeability and the effect of suction.

Cuts in stiff fissured clays will stand up for an undetermined time before failure occurs when inclined steeper than fully softened strength requires. The time may be from six months to hundred or more years. Investigating the stand up time is very complex and requires sophisticated numerical analysis which also utilizes assumptions in many of the parameters used. Very important parameter is the permeability of the clay in the development of swelling and softening.

This is captured in the classical Terzaghi, Peck and Mesri, 1996 book in which it is stated that “If the surfaces of weakness subdivide the clay into fragments smaller than about 25mm, a slope may become unstable during construction or shortly thereafter. On the other hand, if the spacing of the joints is greater, failure may not occur until many years after the cut is made.”

In my opinion the spacing and orientation of joints is the controlling parameter of permeability in such materials. So highly fissured materials with very close spacing may develop shallow slope failures very quickly. More widely spaced  stiff fissured clays with fewer joint systems can provide significant delay time to failure.

A second important issue is the evaluation of shallow or deep seated failure and how a deep seated failure can be defined. This is something very important that will be covered in another entry.

Slope design in stiff fissured clays

Landslide, ,

A very difficult issue is the design of slopes or cuts in stiff fissured clays (pic 1). The difficulty lies in the evaluation of shear strength for stability calculations. Much work has been done on this issue especially by Skempton (1964) with his excellent Forth Ranking Lecture titled: “Long-term stability of clay slopes” and many others have contributed significantly on this issue.stiff fissured clay

In the classical Terzaghi, Peck and Mesri, 1996 book the following is mentioned regarding this issue: “Almost every stiff clay is weakened by a network of hair cracks or slickensides.” (pic.2). “If the surfaces of weakness subdivide the clay into fragments smaller than about 25mm, a slope may become unstable during construction or shortly thereafter. On the other hand, if the spacing of the joints is greater, failure may not occur until many years after the cut is made.”Stiff fissured clay exposed next to a sliding soil mass

The reduction in strength with time, due to the presence of fissures, has been attributed to swelling and softening due to water infiltration in this hairline cracks especially when stress relaxation and crack opening occurs in excavated slopes.

Laboratory shear strength evaluation of such stiff fissured clays is difficult because large samples are required in order to include significant number of hairline cracks and even if such samples can be tested, the long term swelling and softening cannot be fully developed in the laboratory.

Duncan and Wright (2005) propose, based also on the work of Skempton (1970) to use the fully softened strength for long term slope stability evaluation of stiff fissured clays that have not undergone any prior movement or failure. This fully softened strength can be correlated to the peak strength of normally consolidated clays. In the laboratory the fully softened strength is evaluated on remolded samples of stiff fissured clays.

A very recent paper by VandenBerge, Duncan and Brandon, 2013 presents the outcome of a workshop that took place in 2011 at Virginia Tech, regarding the fully softened shear strength for stability of slopes in highly plastic clays.

In this paper the most recent views regarding the softening process, the way to measure or estimate the fully softened strength and how and when to use it in stability analysis are presented. Together with the paper of Vanderberge et al, it is worth reading the Lade paper in Engineering Geology (2010), titled “The mechanics of surficial failure in soil slopes” where a power function failure model is proposed for the shear strength of clay for shallow stability evaluation. This criterion is mentioned also in the paper by VandenBerge et al (2013).

I would like to bring attention on some issues which are mentioned also in the papers but relate more to practical issues of the subject:

  1. Great care should be given when site investigation is executed and evaluated in such stiff fissured caly materialsFissured clay. If core samples are not collected then it is very difficult to distinguish between stiff clay and stiff fissured clay. Even when samples are collected, great care should be given to break up some core samples because the fissuring will not be observed as can be seen in the picture 3.
  2. In situ tests such as SPT will produce high values, misleading the investigator to think that a very strong (even cemented) material is found.
  3. Shear strength tests on intact samples will produce high values of cohesion, again misleading the investigator to believe that a very strong material is present. In the laboratory the fissuring may not be reported during sample preparation due to the small sizes required.
  4. The additional difficulty comes on how to persuade the Owner or Contractor about the problems (failures) that Steep stable excavation in stiff fissured clay may be formed after the slope has been excavated (maybe after very long time). They will evaluate the data from the investigation which show high values and if they are not fully aware about the behavior of stiff fissured clays they will push for a more optimistic design in order to reduce excavation volumes. The situation becomes even more difficult in design – built projects where during excavation the contractor may need to use hydraulic hammers to break up the material and steep slopes are  stable (pic. 4). In such situation everybody “blames” the designer for a very conservative design if fully softened shear strength has been used.
  5. The evaluation of existing stable slopes not designed with fully softened shear strength is another difficult situation. If the fully softened shear strength is used the FS may be found to be even below unity FS<1.0 but the slope is stable for a couple of years after excavation. It is difficult to persuade the Owner of such slopes (usually highway or railway) that in the future stability problems may occur.
  6. Finally it is very difficult to evaluate in what part of the slope and how deep you will use fully softened values especially for high slopes.
  7. Another critical issue is the evaluation of the long term pore pressures to be used in the analysis. In my opinion this is the most difficult issue but I will get back on this in another entry because much could be said.

As a closing remark I would like to state the final conclusion of the Vanderberge et al, 2013 paper: “Consideration of local experience with regard to slope performance, recognition of the possible consequence of slope failures, and application of sound engineering judgment are all essential elements of a comprehensive approach to geotechnical engineering of slopes.”

How difficult is to evaluate superficial landslides – mudflows?

Landslide, ,

Reading on the news about the dramatic landslide-mudflows (picture) that occurred (again) in Petropolis near Rio de Janeiro, with significant fatalities, one thinks could this have been predicted and more so avert it?Brazil mudflow

Even in an area prone to superficial mudflows and thin landslides, it is very difficult to accurately predict the occurrence of such a landslide. This is for the following principal reasons:

  1. It is difficult to assess the geotechnical parameters of a superficial material that undergoes continuous drying and wetting cycles.
  2. The tests are usually performed in saturated samples and in higher effective stresses that are usually present in superficial slides.
  3. A linear Mohr – Coulomb failure criterion is used when it may be more appropriate to use a power function that defines a curved strength envelope with minimum or even zero effective cohesion in very low to zero confinement (especially for uncemented soils).
  4. The soil is usually unsaturated with some (or even large) suction which is modified during rainfall infiltration. The pore pressure regime is very difficult to assess without continuous specific monitoring.
  5. Numerical techniques that can accommodate such complex problems are not widely available and not everybody knows how to use efficiently such models.
  6. The usual triggering mechanism is intense rainfall which increases the pore pressures in the superficial soil.  Since the way water infiltrates in the unsaturated soil, depends mostly on permeability, water content at the time of rainfall, the vegetation cover and the angle of the slope, the problem of evaluating such pore pressures becomes even more difficult.

This problem of predicting mudflows is usually assessed based on prior experience and some statistical evaluation of past mudflows and amount and intensity of rainfall. Can we do better?